Various types of projection lenses are known which are used in video and other projectors having a liquid crystal panel as an image source to modulate a light beam. In this type of projector, light illuminates the liquid crystal display on its back side and passes through liquid crystal picture elements which serve to modulate the light with a picture image that is electrically input to the liquid crystal display. The light is then projected onto a screen by the projection lens, which is arranged after the liquid crystal display so as to form an enlarged image on the screen. Herein, the side of the projection lens on which the liquid crystal display is intended to be located will be referred to as the reducing side, and the side of the lens on which an enlarged image is formed on a screen will be referred to as the enlarging side. Since the percentage of light attenuation in a liquid crystal display serving as a light modulator is comparatively large as a result of the light having to be incident orthogonally onto its back surface for proper operation of the display, it is important that the projection lens be designed with a high optical efficiency so as to not further attenuate the light any more than can be avoided. Therefore, the projection lens should be designed to be nearly telecentric on its reducing side and to have high optical efficiency. In addition, there is strong market demand for the entire device to be light weight as well as compact. Therefore, small scale lens elements as well as the use of a small number of lens elements is desired in such a projection lens.
Japanese Laid-Open Patent Application 7-294809 and U.S. Pat. No. 2,687,063 disclose conventional projection lenses which have a relatively low F-number, and wherein various aberrations are favorably corrected despite the projection lens having only a small number of lens elements.
The projection lens described in Japanese Laid-Open Patent Application 7-294809 has a relatively bright image, in that its F-number value is about 2.0. However, there is peripheral darkening in that the projection lens is not telecentric on its reducing side, with the maximum angle of a principal ray relative to the optical axis (hereinafter termed the telecentric angle) on the reducing side of the projection lens being as large as 6 degrees.
The projection lens described in U.S. Pat. No. 2,687,063 forms a brighter image, in that its F-number is 1.7. However, again there is unnecessary peripheral darkening since the telecentric angle may be as large as 5 degrees. Furthermore, the distortion is too large for this lens to be used as a projection lens in state-of-the-art projectors.